The production of non-woven webs involves the dry forming of fibrous materials, such as dry fibers, filaments, and particulate matter onto a moving forming surface. In systems for dry-forming fibrous materials, critical process limitations have been found to exist in systems where the speed of the forming surface increases to greater than 500 feet per minute. At such high speeds, fiber lay-down on the moving forming surface tends to become uneven in the machine direction. The deposited webs exhibit an upper surface having an undulated, wave-like or ripple effect extending in the cross-machine direction and the webs exhibit corresponding variations in thickness, and basis weight. The rippling effect worsens with increasing speed of the forming surface, and eventually renders the web commercially unacceptable when a certain high range of speed of the forming surface is used.
Commonly assigned U.S. Pat. No. 4,276,248 to Widnall describes the problems associated with the formation of dry laid non-woven webs, particularly at wire speeds of greater than 500 feet per second and offers some solution to these problems. As disclosed therein, a critical fiber velocity relationship exists which can alleviate these detrimental wave characteristics in the web. This relationship, hereinafter "formation ratio", provides that the magnitude of the difference between the foraminous wire or web horizontal velocity and the fiber horizontal velocity component divided by the vertical velocity component of the fibers should be less than 3.0, preferably less than 2.5.
Attempts to alleviate the above-described problems are described in U.S. Pat. No. 4,264,290 to Dunkerly et al. (herein "Dunkerly et al.") and U.S. Pat. No. 4,285,647 to Dunkerly (herein "Dunkerly"), both commonly assigned. These patents illustrate certain means for inducing a horizontal velocity component to dry-laid fibers. Dunkerly et al. and Dunkerly show that a suction box beneath a moving foraminous wire may be offset in the machine or downstream direction to induce a horizontal velocity to the dry-laid fibers. That is, the upstream wall of the suction box lies beneath the distributor and is displaced by a finite distance from the distributor upstream wall, while the downstream suction box wall extends beyond the distributor to draw fiber-laden air in the direction of the moving wire.
Dunkerly et al and Dunkerly also show additional means for inducing a horizontal component to dry-laid fibers to augment the effect of offset suction. Dunkerly et al teaches use of a plurality of foils directing air horizontally into the gap between a fiber distributor and forming wire. Dunkerly illustrates various vane and deflector arrangements directing air horizontally within a fiber distributing system above the forming wire.
The present invention provides new means for accelerating fibers in the horizontal direction to alleviate the aforementioned problems to produce a uniform dry-laid web at high speeds, and may be used alone or in combination with existing systems.